Highlights
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Careful patient selection, meticulous microsurgical technique, refined surgical strategy, and high-volume operative experience with arteriovenous malformations is key to prevent complications during microsurgical arteriovenous malformation resection.
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Intraoperative complications need to be addressed directly. When unintentional residual arteriovenous malformation is seen on postoperative angiography, reoperation is recommended within 48 hours.
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Arteriovenous malformation steal phenomenon caused by reduced blood flow in the surrounding healthy cortex is rare and can cause seizure, stroke, or other focal neurologic deficits, which may resolve after arteriovenous malformation resection.
Background
The microsurgical resection of an arteriovenous malformation (AVM) is a technically challenging endeavor. However, surgical resection remains the treatment modality with highest rates of cure and low rates of complication when compared with other treatments—including endovascular treatment or radiosurgery, despite the currently published ARUBA trial. Careful patient and AVM subtype selection, meticulous microsurgical technique, precise surgical strategy, and surgical experience as well as a high AVM surgical volume are needed to prevent complications and to provide a favorable patient outcome. Based on Spetzler-Martin (SM) grading and the supplementary SM grading system, patients are classified into subtypes and are risk stratified to effectively select appropriate patients for surgery. In a multicenter validation study, the supplemented SM grading system was more precise than the SM grade for patient selection for surgery, with an acceptable surgical risk at 6 points or below. The book Seven AVMs further describes how to subgroup AVMs based on locations and provides a nuanced stepwise approach to guide microsurgical resection to prevent complications and to achieve a complete AVM resection. Despite adequate preparation and experience, unexpected things can happen during AVM surgery, requiring quick recognition and decisive action. Complications in AVM surgery include intraoperative AVM rupture, incomplete surgical resection with/without hemorrhage, and injury or occlusion of nonfeeding arteries causing stroke with/without clinical deficits, as well as delayed hemorrhage due to fragile deep feeding arteries. These complications may occur immediately or may present in a delayed fashion. The rarely described AVM steal phenomenon due to reduced blood flow in the surrounding healthy cortex can cause seizure, stroke, or other focal neurologic deficits. This can also occur after AVM resection and might be due to the reorganization of blood distribution.
Anatomic Insights
Based on the SM grade or supplementary grade, the location of AVM (eloquent vs noneloquent), type of venous drainage (deep or superficial), size of the AVM (small or large), and additional features including nidus type (compact vs diffuse), patient age, and the presence of prior AVM-related hemorrhage are factors influencing the likelihood of complications and patient morbidity during or after AVM surgery. AVMs in eloquent locations—including the brainstem, thalamus, or primary motor or language cortices—are more prone to complications than those in other locations. This is in part due to brain retraction or misinterpretation and to sacrifice of feeding arteries with perforators, which may lead to acute infarction ( Fig. 11.1 ).
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Complications will most likely happen if the draining vein is occluded too early. Draining veins should always be preserved until the end of the operation and should be sacrificed only in proportion to disruption of the arterial feeders.
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AVM border dissection too close to the nidus can result in residual AVM nidus and/or intra-/postoperative hemorrhage. Make sure to have enough distance to the nidus in noneloquent AVMs.
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Keep anesthesia involved: Always check blood pressure and estimated blood loss to stay on top of ongoing losses and remain one step ahead of the pathology.
Risk Factors
Risk factors for complications in AVM surgery can be grouped into patient factors, AVM characteristics, and AVM location, which were described in the preceding subsection. Patient factors include age and cardiovascular and other medical comorbidities, which affect a patient’s general surgical risk. Antiplatelet medication, anticoagulants, or conditions leading to an impaired coagulation increase the risk of hemorrhage perioperatively. High-risk AVM characteristics include the presence of aneurysms in the feeding arteries, nidus, or venous drainage. A large venous varix can mask the nidus or feeding arteries, and these dilatations need to be carefully dissected to prevent rupture. Other high-risk features are stenosis or narrowing of the draining vein, which can increase the pressure in the AVM nidus. AVMs reaching the ventricle often have small, fragile subcortical feeding arteries, which are difficult to coagulate and need to be clipped with small AVM/aneurysm clips.
Prevention of Complication
Preoperative Prevention
Preoperative catheter angiography, magnetic resonance image (MRI)/MR angiogram, and/or computed tomography (CT) angiogram must be carefully evaluated to plan the surgical approach to maximize AVM exposure and minimize transgression of normal brain. The complex 3-dimensional angioarchitecture of the AVM, including the feeding arteries, nidus, and draining veins, must be carefully studied and high-risk features identified. A large craniotomy is warranted so that all of these features also may be confirmed and visualized intraoperatively. In addition to AVM-specific preoperative planning, all patients should undergo thorough medical evaluation to ensure safety with general anesthesia, cardiovascular stress, and the avoidance of coagulopathic conditions.
Perioperative Prevention
Patient positioning depends on AVM location by using standard non–skull-base and skull-base approaches if necessary. Surgical adjuncts including MR navigation, indocyanine green (ICG) angiography, and electrophysiologic monitoring are useful during AVM surgery. MR navigation can help plan the approach that minimizes transgression of normal brain and to provide guidance during the resection. Intraoperative electrophysiology, including motor evoked potentials (MEPs) and somatosensory evoked potentials (SEPs), provides valuable information to prevent strokes before definitive occlusion of vessels during surgery. Surgical resection should follow a systematic and stepwise surgical approach to standardize the resection without missing important aspects. After large exposure, one must first identify feeding arteries and draining veins through careful subarachnoid dissection both to appreciate the 3-dimensional configuration of the lesion and to identify surgical planes. Wide pial dissection should be conducted in circumferential fashion around the AVM. During dissection, feeding arteries should be divided when encountered and draining veins preserved. After a large exposure of the AVM, the feeding artery/ies and draining vein/s are identified by subarachnoid dissection, which helps the surgeon to visualize the AVM nidus and surgical planes. While dividing the feeding arteries, the AVM borders are carefully encountered using a parenchymal circumferential dissection. The bottom border is always the last part to dissect, since deep small feeding arteries that may be difficult to control may be encountered. Therefore it is important to have enough space around the AVM to address these feeders to avoid intraoperative arterial AVM rupture. In a last step, the draining vein is cauterized, and the AVM is resected. ICG angiography might be useful to show AVM nidus residual or an early-filling vein as an indirect sign for AVM residual. Postoperative blood pressure should be kept in the low-normal range for 24 hours to prevent postoperative hemorrhage into the resection cavity.
Management
In general, the earlier the better is the rule for the management of AVM surgical complications—including postoperative hemorrhage and residual nidus. Intraoperative AVM rupture leads to a change of surgical strategy: An AVM resection needs to be finished as soon as possible to avoid blood loss. A take-back is painful for both the patient and the surgeon, but it is the treatment of choice in residual AVMs. Postoperative angiography is the gold standard to rule out residual AVM nidus and should be performed within 24 hours after surgery. If postoperative hemorrhage is suspected based on clinical examination, a noncontrast CT scan should be ordered. New neurologic deficits after surgery without a cause shown on CT or catheter angiography need MRI with diffusion-weighted imaging and/or electroencephalography to rule out postoperative stroke or seizure, respectively. In most cases direct postoperative neurologic deficits are only temporary due to steal phenomenon or irritation during surgery. If a postoperative stroke is identified, standard medical therapy should be initiated—including permissive hypertension—to facilitate restoration of regional cerebral perfusion loss.
Microsurgical Management of Residual AVM Nidus
In the case of a clearly documented AVM residual, the patient should be offered reoperation within 48 hours after the initial surgery. This is advantageous because the initial craniotomy is opened quickly without scar tissue, and the residual AVM nidus can be reached more easily than at later time points. Sometimes it is possible that postoperative catheter angiography does not show a true residual nidus but instead suspicious small vessels without a clear draining vein. In this situation it is reasonable to repeat an angiogram after 4 to 6 weeks to let the postoperative changes resolve. If there is evidence for an AVM residual on the follow-up angiography, reoperation should be considered.
Microsurgical Strategy in Intraoperative AVM Rupture
Intraoperative AVM rupture is an uncommon complication, occurring with a 5% frequency, and can be caused by either rupture due to arterial bleeding, nidal penetration, or premature venous occlusion. Depending on the extent of hemorrhage, it is possible in some cases to control small amounts of bleeding with coagulation or clips, such as with small, unintended nidal penetration. In other scenarios with brisker rates of bleeding, such as early venous occlusion with AVM rupture, a change in surgical strategy must be implemented, and the AVM must be resected as quickly as possible to stop ongoing losses. No matter how severe the intraoperative AVM rupture is, it needs to be addressed immediately; otherwise, intraoperative AVM rupture can be devastating and ultimately fatal. The so-called “commando resection” is an option of last resort when one is faced with impending or frank AVM rupture. It is not meant for simple arterial or venous bleeding or for minor nidal bleeding. Rupture of an AVM calls for decisive action, and the commando resection represents a point of no return. Measures explained in the prior subsections should be implemented to avoid intraoperative rupture.
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